This invention relates to electro-optic devices and, more particularly, to proximity coupled light valves for electro-optic line printers and the like.
It has been shown that an electro-optic element having a plurality of individually addressable electrodes can function as a multigate light valve for line printing. See, for example, U.S. Pat. No. 4,281,904 on a "TIR Electro-Optic Modulator with Individually Addressed Electrodes," which issued Aug. 4, 1981 on an application of R. A. Sprague et al. Also see, "Light Gates Give Data Recorder Improved Hardcopy Resolution," Electronic Design, July 19, 1979, pp. 31-32; "Polarizing Filters Plot Analog Waveforms," Machine Design, Vol. 51, No. 17, July 26, 1979, p. 62; and "Data Recorder Eliminates Problem of Linearity," Design News, Feb. 4, 1980, pp. 56-57.
As is known, almost any optically transparent electro-optical material can be used as the electro-optic element of such a light valve. The most promising materials now appear to be LiNbO.sub.3 and LiTaO.sub.3, but there are others which merit consideration, including BSN, KDP, KD.sup.x P, Ba.sub.2 NaNb.sub.5 O.sub.15 and PLZT.
To control the light valve, the electrodes are proximate the electro-optic element and are distributed widthwise thereof, typically on equidistantly spaced centers. For certain applications, such as high resolution line printing, the electrodes are densely packed on centers of, say, ten microns or even less. Fortunately, it has been found that the electrical interface for such a light valve can be significantly simplified if the electrodes are fabricated on a separate substrate, such as a silicon integrated circuit, and pressed or otherwise held closely adjacent the electro-optic element to "proximity couple" electric fields into the electro-optic element. See, a commonly assigned U.S. Pat. No. 4,396,252 of W. D. Turner, which issued Aug. 2, 1983 on "Proximity Coupled Electro-Optic Devices," and a commonly assigned U.S. Pat. No. 4,367,925 of R. A. Sprague et al., which issued Jan. 11, 1983 on "Integrated Electronics for Proximity Coupled Electro-Optic Devices."
However, experience with proximity coupled multigate light valves has demonstrated that it is difficult to obtain highly efficient and uniform coupling. The inherent surface roughness of the electrodes, minor localized defects in the mating surfaces of the electrodes and the electro-optic element, and even dust particles or other particulates entrapped between those surfaces tend to create an irregular gap. A commonly assigned U.S. Pat. No. 4,482,215 of R. A. Sprague et al, which issued Nov. 13, 1984 on "Mechanical Interface for Proximity Coupled Electro-Optic Devices," suggests mechanically gapping the electrodes a predetermined nominal distance from the electro-optic element, thereby reducing the effect of the gap irregularities on the uniformity of the fields that are coupled into the electro-optic element. Furthermore, another commonly assigned U.S. Pat. No. 4,370,029 of R. A. Sprague et al., which issued Jan. 25, 1983 on "Dielectric Interface for Proximity Coupled Electro-Optic Devices," suggests filling the the gap with a material having a high dielectric constant to provide even more uniform and efficient coupling. In short, these prior proposals have acknowledged the existence of a gap between the electrodes and the electro-optic element and have attempted to reduce its adverse effects.